The present invention relates to an enamel composition for producing a reflecting white dielectric layer in plasma display panels. The invention further relates to white pigments with improved wettability which pigments are present in the enamel composition. The invention still further relates to a process for enamelling a glass substrate for producing a reflecting dielectric layer with improved properties in plasma display panels and plasma display panels containing said reflecting dielectric layer.
A plasma display panel generally comprises two opposed glass substrates, electrodes systematically arranged in the glass substrates and a noble gas there between. More particularly a plasma display panel (PDP) comprisis a first array of electrodes embedded in a dielectric layer on the rear glass substrate, a second array of electrodes embedded in a dielectric layer on a front substrate and a pattern of a barrier for defining discharge spaces in between.
The structure and a process for producing a plasma display panel (PDP) is described in the U.S. Pat. No. 5,909,083. This document also discloses the composition of transparent and pigmented enamels such as used for a primer layer to be disposited on glass substrate, a dielectric layer which covers the address electrode and several compositions for producing the barrier system. The dielectric layer forming constituants essentially comprise a glass frit, based on e.g. Bi2O3, SiO2, ZnO and B2O3, and a pigment like titanium dioxide and in addition thereto aluminium oxide. The mixture of said inorganic ingrediences has a softening temperature of 570xc2x0 C., and the fired enamel has a coefficient thermal expention xcex1=80xc2x710xe2x88x927 Kxe2x88x921. For screen printing purposes the said enamel composition further contains a medium comprising a resin binder and an organic solvent system.
The U.S. Pat. No. 5,948,537 relates to a non-cristylizable low melting glass compositions based on 52 to 68 wt.-% of PbO+Bi2O3, 14 to 28 wt.-% of B2O3 and 6 to 23 wt.-% of ZnO and facultatively small amounts of SiO2, Al2O3, CeO2 and SnO2. Said glass composition has a softening point of at most 510xc2x0 C. and a thermal expansion coefficient xcex120300 of 70 to 85xc2x710xe2x88x927 Kxe2x88x921. This document is silent about the use of pigments within the dielectric layer. Other glass compositions for dielectric glass layers in PDP""s consist of (wt.-%) 55 to 70 PbO, 6 to 25 B2O3, 6 to 25 SiO2, 1 to 10 ZnO, 1 to 5 K2O, Cr2O3, CuO, NiO, MnO, CoO or vanadium oxidexe2x80x94see JP 10208644 A. A glass frit on the basis of a lead borosilicate containing (wt.-%): 62,4 to 69,6 PbO, 5,8 to 13,6 SiO2, 13,6 to 20 B2O3, 0,2 to 1 Al2O3, 0 to 5 MgO and 0 to 6 CaO for dielectric layers for PDP""s is tought by JP-A 50-23414.
A glass frit composition for a dielectric layer in PDP""s must fit several kind of specifications: Physical specifications like high compatability with glass panels, an essentially complete ( greater than 99%) coating at a firing temperature of preferably 550 bis 580xc2x0 C.; optical specification like a reflection coefficient as great as possible to improve the luminance; electrical specification like a break down voltage greater than 400 V/25 xcexcm. The glass frit compositions of the above cited documents do not fully meet the required properties in the one or other aspect. This happens especially in those cases where the glass composition contains a white pigment for providing the dielectric layer with a high reflection coefficient. One important problem the skilled has to cope with in using enamel compositions containing white pigments for producing dielectric layers for PDP""s is the porosity of said layers obtained by firing. The high porosity is most probably caused by the bad wetting properties of the pigments by the glass matrix. The quality of the reflecting dielectric layer for PDP""s can be checked by light microscope observation and by sweep electron microscope observationxe2x80x94insufficient quality is characterized by an uneven surface aspect, a high number of bubbles and inhomogeneity of the micro structure.
It is a first object of the present invention to provide an enamel composition for producing a reflecting white dielectric layer for plasma display panels with improved properties, like especially a very low number of bubbles within the fired dielectric layer. Further objects of the invention are directed to provide a method for improving the wettability of white pigments for using them in an enamel composition for use in PDP""s and a process for the production of glass layers with a porosity which is significantly lower than obtained by using conventional pigments. Further object of the invention can be seen from the following disclosure.